The present invention relates in general to wireless communication systems and, more particularly, to using antenna array and signal processing techniques to increase downlink capacity and performance of wireless communication systems. The present invention relates to a new technique for generating downlink beamforming weights for frequency division-duplex (FDD) communication systems, which may include time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) and their combinations.
The next generation of wireless mobile communication systems will be required to provide a variety of services including digital voice, video and data in different transmission modes. These services will require high rate data transmission and high received signal power level, thus creating increased interference among users. In order to obtain high system capacity and reliable transmission quality, the interference level has to be reduced dramatically. Spatial division multiple access (SDMA), by which a plurality of antenna elements are equipped at the base station in order to receive and transmit data information from and to the desired user by using spatial diversities, has been proposed as an effective technique to reduce this kind of interference.
The main operations in SDMA include uplink (from mobile station to base station) beamforming and downlink (from base station to mobile station) beamforming. Uplink beamforming consists of uplink beamforming weight generation and uplink signal demultiplexing; Downlink beamforming includes downlink beamforming weight generation and downlink signal multiplexing. Theoretically, in both links, the associated channel responses are of critical importance in order to generate the corresponding beamforming weights.
Whilst much attention has been paid to uplink capacity enhancement, it is also desirable to improve downlink performance in order to increase the whole system capacity. Moreover, downlink capacity is even more important for the next generation of communication systems in which wireless internet, video-on-demand and multimedia services are to be implemented.
There are many different configurations of antenna array, of which the most popular and widely used one is uniform linear array (ULA), due to its easy implementation, and simplicity in estimating direction-of-arrival (DOA) information.
There are two duplex multiplexing modes for wireless communication systems: time-division-duplex (TDD) mode, and frequency-division-duplex (FDD) mode. For TDD mode, uplink and downlink employ the same carrier frequency, but are assigned different time slots. This mode makes downlink beamforming easy, but requires accurate synchronization between uplink and downlink transmissions. To this end, a guard time period is usually inserted, which may greatly degrade the transmission efficiency. In FDD mode, on the other hand, both links use different carrier frequencies, thus the transmissions in these two links are independent, and no coordination is required. The FDD scheme is adopted in most current wireless communication systems, and most probably will be used in the next generation systems. For FDD systems, there is no direct measurement of downlink channel responses at the base station, which makes downlink beamforming more difficult than its uplink counterpart. One method for simplifying the downlink beamforming problem is based on the use of direction-of-arrival (DOA) information embedded in received uplink signals. In fact, since uplink and downlink signals travel through reflections and deflections due to the same scatters surrounding the mobile and the base station, the DOAs of the uplink signals might be the only constant parameters which can be used for solving the downlink beamforming problem.
DOA-based approaches employ the received uplink signals to compute the desired user's DOAs first; then downlink channel responses are constructed using the existing relationship between uplink and downlink steering vectors; finally downlink beamforming weights are chosen to be the same as the corresponding downlink channel responses. In WO 97/45968, “Method of and apparatus for interference rejection combining and downlink beamforming in a cellular radiocommunications system”, Forssen et al proposed to compute the probability function with respect to different DOAs at which the desired signal may come from, and to choose the angle of incidence associated with the particular mobile station as the DOA value which maximises the probability function. This technique, however, suffers from a heavy computational burden in computing the probability function and searching for the maximum point. In WO 96/22662, “Spectrally efficient high capacity wireless communication systems”, Barratt et al use subspace-based techniques (e.g., MUSIC and ESPRIT) to obtain high-resolution DOA estimates from the covariance matrix of the antenna outputs. It is well known that subspace-based algorithms require very complicated computations since they are involved in the computation of matrix inversion or singular value decomposition of complex matrices, and one or even more multidimensional non-linear optimisations. Unfortunately, accurate DOA estimates are not available in multipath cases since the number of multipath DOAs are usually greater than the number of antenna elements. This may limit the applicability of the DOA-based approaches for downlink beamforming in wireless communication systems. Further, DOA-based approaches are based on the promise of keeping the main beam of the downlink beam pattern directed toward the intended user, but do not take into account the interference pollution to other users who are not intended to receive the signal, thus such DOA-based approaches are far from the optimal solution in terms of system capacity.
If downlink channel responses are really available, F. Rashid-Farroki et al proposed a virtual uplink beamforming and power control technique (UBPCT) to generate downlink beamforming weights for TDMA and CDMA without multipath case, see “Transmit beamforming and power control for cellular wireless systems”, IEEE Journal of Selected Areas in Communications, vol. 16, No. 8, 1998, pp. 1437-1449. In Singapore patent application, No. 9904733.4, entitled “Downlink beamforming method for capacity enhancement in wireless communication systems”, filed on 24 Sep. 1999, Y. Liang et al proposed a modified UBPCT, which is suitable for direct sequence code division multiple access (DS-CDMA) with multi-delay paths and considering the orthogonality of downlink codes as well as the multi-rate services. The virtual UBPCT methods are based on the criterion of minimising the total transmitted power while maintaining the SIR requirements, which is physically meaningful from the view point of downlink power limitation, as well as of minimising the interference pollution to the outer cells, thus of increasing the multi-cell capacity. Therefore, the downlink beamforming weights generated by the virtual UBPCT are actually the optimal solutions of downlink beamforming problem in terms of capacity enhancement.
Virtual UBPCT is more computationally efficient than the traditional downlink beamforming approach in the sense that no multi-variable optimisation is required. However, this method still requires matrix inversion and some iterative computations. More seriously, this method requires the downlink channel responses, which can be difficult to obtain at the base station.